This invention relates to magnetic recording media, such as thin film magnetic recording disks, and to a method of manufacturing the media. The invention has particular applicability to high areal density longitudinal magnetic recording media having very low medium noise, and more particularly, to media having a continuously varying composition in the underlayer for improved lattice spacing match and crystallographic orientation.
Magnetic discs and disc drives provide quick access to vast amounts of stored information. Both flexible and rigid discs are available. Data on the discs is stored in circular tracks and divided into segments within the tracks. Disc drives typically employ one or more discs rotated on a central axis. A magnetic head is positioned over the disc surface to either access or add to the stored information. The heads for disc drives are mounted on a movable arm that carries the head in very close proximity to the disc over the various tracks and segments.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), remanent coercivity (Hr), magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material, coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. These parameters important to the recording performance and depend primarily on the microstructure of the materials of the media. For example, as decreasing the grain size reduces the SMNR or reducing exchange coupling between grains, it has been observed that the thermal stability of the media decreases.
The requirements for high areal density, i.e., higher than 30 Gb/in2, impose increasingly greater requirements on magnetic recording media in terms of coercivity, remanent squareness, medium noise, track recording performance and thermal stability. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high-density magnetic rigid disk medium for longitudinal and perpendicular recording.
As the storage density of magnetic recording disks has increased, the product of Mr and the magnetic layer thickness t has decreased and Hr of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hr. To achieve a reduction in Mrt, the thickness t of the magnetic layer has been reduced, but only to a limit because the magnetization in the layer becomes susceptible to thermal decay and medium noise.
Medium noise in thin films is a dominant factor restricting increased recording density of high-density magnetic hard disk drives, and is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Longitudinal magnetic recording media containing cobalt (Co) or Co-based alloy magnetic films with a chromium (Cr) or Cr alloy underlayer deposited on a non-magnetic substrate have become the industry standard. For thin film longitudinal magnetic recording media, the desired crystallized structure of the Co and Co alloys is hexagonal close packed (hcp) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis that lies in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable is the Co alloy thin film for use in longitudinal recording to achieve high remanance and coercive force. For very small grain sizes coercivity increases with increased grain size. The large grains, however, result in greater noise. Accordingly, there is a need to achieve high coercivities without the increase in noise associated with large grains. In order to achieve low noise magnetic recording media, the Co alloy thin film should have uniform small grains with grain boundaries capable of magnetically isolating neighboring grains thereby decreasing intergranular exchange coupling. This type of microstructural and crystallographic control is typically attempted by manipulating the deposition process, and proper use of underlayers and seedlayers.
Underlayers can strongly influence the crystallographic orientation, the grain size and chemical segregation of the Co alloy grain boundaries. Underlayers include Cr and alloys of Cr with elements such as titanium (Ti), tungsten (W), molybdenum (Mo) and vanadium (V). It is recognized that the magnetic properties, such as Hcr, Mr, S and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer, which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particularly for growing fine grains of hcp Co alloys deposited thereon.
Goda et al. in U.S. Pat. No. 5,766,756 disclose a magnetic recording medium comprising a glass ceramic substrate and separately deposited chromium underlayers that can also comprise alloys of chromium, such as chromium tungsten. Lal et al. in U.S. Pat. No. 5,569,533 disclose a magnetic recording medium having an underlayer system comprising a first chromium or chromium alloy underlayer and a second chromium or chromium alloy underlayer on the first underlayer. Lal et al. in U.S. Pat. No. 5,456,978 disclose a magnetic recording medium comprising a chromium-containing sublayer interposed between the substrate and a chromium underlayer. Ivett et al. in U.S. Pat. No. 5,298,324 disclose a magnetic recording medium comprising a chromium-tungsten underlayer.
Cr/CrW double underlayer structure was disclosed in Ser. No. 09/497,524, which discloses a longitudinal media design in which two chromium alloys are coupled uni-directionally. The structure allows enhanced breakup of intergranular exchange coupling. This, in turn, affords media designers the ability to utilize high anisotropy, high magnetization materials that are generally also highly exchange coupled in their multilayer structure. Despite the advantages of the Cr/CrW double underlayer structure, Applicants found that the double underlayer structure results in the formation of the Cr and CrW interface which is prone to contamination as well as stress from lattice mismatch.
There exists a continuing need for high areal density longitudinal magnetic recording media exhibiting high Hcr and high SMNR.
The invention discloses bi-crystal magnetic recording media comprising a Cr and W containing underlayer possessing a strong crystallographic texture and a favorable lattice spacing match with the magnetic alloy of the magnetic recording layer. The lattice spacing match is achieved by varying the composition of W in the thickness direction of the underlayer. Therefore, one embodiment comprises an underlayer initially having substantially pure Cr and then having an increasing amount of W with increasing film thickness formed by co-sputtering, thereby eliminating a Cr and CrW interface. Such an underlayer has a fine grain size structure with strong (200) orientation as well as a large lattice constant by incorporating substantial amount of W near the top of the underlayer. Another embodiment relates to a method of manufacturing the magnetic recording medium containing a continuously varying composition in the underlayer for improved lattice spacing match and crystallographic orientation.
In this invention, xe2x80x9cmeans for obtaining improved lattice spacing match of the magnetic recording layerxe2x80x9d refers to a Cr and W containing underlayer having an increasing amount of W with increasing film thickness or equivalents thereof.
As will be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.